The invention relates generally to pointing devices and relates more particularly to pointing devices having Z-axis functionality.
With the widespread adoption of graphical user interfaces, it has become very important to provide pointing devices so that users can provide inputs that cause motion of a cursor on a screen. While mice and other rodents are in common use, many users now prefer stick pointers and touch pads, typically integrated into a computer. The typical stick pointer is a button located in the middle of the keyboard, connected mechanically to a strain gauge. Circuitry detects signals developed in the strain gauge to derive X and Y information regarding lateral force applied to the gauge by the user through the button.
The typical strain gauge is essentially a resistor bridge, the resistances of which are affected by forces in the bulk material of the gauge. Assuming that cost, power consumption and physical bulk are no object, then it is a straightforward matter to provide excitation signals to nodes of the bridge and to detect voltages at other nodes of the bridge. The detected voltages can be provided to one or more analog-to-digital convertors of arbitrarily high speed, accuracy and precision for further processing.
A stick pointer sensor is conveniently connected to its electronics by means of a flexible sensor cable, which offers the great advantage that the sensor and the electronics do not need to be close to each other. Such a cable is, however, a significant contributing source of noise and drift in the system, due to its relatively great length (several inches) and, therefore, the ability to pick up noise and RFI (radio frequency interference) from nearby circuitry and from other sources such as cellular telephones.
For the designer of circuitry attached to the gauge, however, there are many competing design goals. It is desired to minimize component count outside of any integrated circuits, to minimize assembly cost. It is desired to have immunity to any of a number of sources of noise and drift. In recent years, as computers have become smaller and lighter in weight, it is important to make the circuitry small and light in weight. The trend toward smaller and lighter computers also prompts the computer designer to minimize battery size and weight, thus forcing the pointing device circuitry designer to try to minimize power consumption Despite all of these constraints it is also desired that the pointing device be very accurate. Approaches for such pointing devices may be seen in U.S. Pat. No. 6,175,359, issued Jan. 16, 2001, and U.S. Pat. No. 5,874,938, incorporated herein by reference.
These constraints and goals make the design of the pointing device system very challenging. One skilled in the art may then appreciate that yet another design goal has presented itself in recent yearsxe2x80x94allowing the stick pointer to perform not only as a pointer (provider of X-Y data) but also as a left mouse button (provider of Z data). When this design goal is added to the many goals and constraints discussed above, the design task is extremely challenging.
It is desirable, then, to provide a pointing device having reduced component count, Z-axis functionality, with acceptable noise rejection and signal-to-noise ratio, programmable gain, and high isolation from power supply noise.
Apparatus and method are disclosed for receiving human input by means of a pointing device, typically a strain gauge. Three dimensions of input are received from a resistor bridge by means of four contact points to the strain gauge. A sequence of excitation signals is applied to the bridge and a multiplexer is employed to permit a signal processor to sample sensor outputs. The apparatus processes the sampled outputs and determines the user inputs such as X and Y and Z values of the pointing device.